1. Field of the Invention
This invention relates generally to protective packages suitable for use with semiconductor integrated circuit devices, and relates more particularly to an apparatus and a method for forming a protective package for an EPROM circuit.
2. Description of the Prior Art
One common type of integrated circuit device is known as an erasable programmable read-only memory (EPROM). An EPROM circuit is a memory device that can be reprogrammed. A typical EPROM memory cell has a floating gate that stores a charge to remember one bit of information. To erase this type of memory cell, the charge is drained from the floating gate by exposing the EPROM circuit to radiation, such as ultraviolet light. Once erased, the memory cells of the EPROM circuit can be reprogrammed by supplying additional charges to their floating gates. A radiation transparent window must be provided in the protective package to allow the EPROM circuit to be exposed to radiation.
Integrated circuit chips are commonly housed in protective packages to protect the fragile chips, and to provide convenient packages for handling and interconnection with other electronic circuits. One type of device package from EPROM circuits was disclosed in U.S. Pat. No. 4,326,214 issued on Apr. 20, 1982 to R. K. Trueblood and entitled "Thermal Shock Resistant Package Having an Ultraviolet Light Transmitting Window for a Semiconductor Chip". The Trueblood Patent disclosed a two piece protective package composed of a ceramic material. The protective package included top and bottom pieces, with a glass window bonded to the top piece. To form a protective package the chip was bonded to the bottom piece and wired to a lead frame. Then the top piece was positioned over the chip and was bonded to the bottom piece and lead frame to hermetically seal the chip.
The major disadvantage to the protective package disclosed by Trueblood was its cost. Large ceramic parts are costly to fabricate due to the brittleness of the material.
Integrated circuit chips other than EPROM's have been encapsulated in plastic to provide a low cost protective package. Using this technique, a chip is first wired to a lead frame, then the chip and lead frame are placed in a cavity in a mold and plastic is injected into the cavity. The plastic flows around the chip and lead frame to fill the cavity, and, when cooled, forms a protective package.
Such a technique has been found to be unsuitable for use with EPROM chips because the top surface of a chip must be visible through a radiation transparent window. If a molded plastic protective package is to be used with EPROM chips, isolation means must be provided to isolate the top surface of the chip from the mold cavity to prevent the plastic from covering the top surface of the chip. A major disadvantage to such an approach is that the chip and its wiring are very fragile and susceptible to damage from the isolation means.